The perfect sandwich cookie should be crisp and crunchy on the edges but ever so slightly soft in the center, so it doesn't shatter too much around the filling. This one is just right. The dough is quite soft and sticky, so a quick chill always helps make it easier to roll out if you're having trouble. You can fill it with a simple buttercream if you want something more basic, or use one of my three favorite fillings: tahini, peppermint, or matcha. —Posie (Harwood) Brien
2 dozen sandwich cookies
(113 grams, 1 stick) unsalted butter
(132 grams) granulated sugar
black cocoa powder (you can use all regular cocoa if you prefer; this will just give a more intense color)
In the bowl of a stand mixer, beat together the butter, oil, and sugar until pale and fluffy. Add the vanilla and egg and beat well.
Whisk together the remaining ingredients and add to the wet ingredients. Mix until just combined.
Chill the dough for about 20 minutes--this isn't crucial for flavor but does help to roll out the cookies.
Preheat the oven to 350 degrees F. Roll out the dough to about 1/4" thick (working with half the dough at a time will keep it from warming up too much). If the dough is too sticky, dust your work surface with cocoa powder.
Using a round or scalloped edge cookie cutter, cut out cookies and place them on a parchment-lined baking sheet. Bake the cookies for 10-12 minutes -- they should be just set and should smell like chocolate. Remove from the oven and let cool; the cookies will seem soft at first but will crisp as they cool.
Let the cookies cool fully before filling them with your favorite filling.